Asymmetrical location of the external auditory meatuses and lateralization

Abstract:

Since the face forms over the brain in the course of embryonic development, facial anthropometry may reflect brain structure. The motor functions of each side are controlled by the side of the brain opposite the body side. The purpose of this study was to establish whether a correlation exists between handedness and the location of the external auditory meatuses, as a possible consequence of brain asymmetry. Facial photographs were taken of 78 volunteers. Straws, placed in the external ear canals, were used to mark the external auditory meatuses. The level of the top of each meatus was measured from each volunteer's visual plane, as established by connecting the center of a point of reflected light in each pupil. Each volunteer was also given the Edinburgh Laterality Inventory (Durden-Smith and DeSimone, 1984:53) to determine "true" handedness (50 right-handers and 28 left-handers). Right-handers, as determined by either writing hand or laterality inventory, were found to exhibit a greater tendency for the left auditory meatus to be lower. Specifically, 68% of the right-handers, as opposed to 39% of the left-handers, exhibited a left external auditory meatus located at a lower position on the skull than the right meatus. This was significant at the 0.05 level. The differences in external auditory meatal distances from the visual plane were greater on the left in right-handers 68% of the time, equal 10%, and greater on the right 22% of the time. A reverse correlation for the right asymmetry for left-handers was not found. Instead, for the left-handed sample a nearly even distribution for meatal location was found: 39% left asymmetry, 29% symmetry, and 32% right asymmetry.The study strongly supported the hypothesis that right-handers have a significant tendency for left asymmetry in location of the external auditory meatuses. The study did not support the hypothesis that the meatal asymmetry correlates to the side opposite the handedness of the individual. Of-perhaps greater significance is the finding that the percentages of left asymmetry of both groups match the brain asymmetry percentages found by Galaburda (1984:15) for the planum temporale, an extension on the upper surface of the temporal lobe of the brain. The level of the external auditory meatuses, as a reflection of brain asymmetry, may serve as an external measurement of the location of Wernicke's area which is located near the planum temporale and has a major role in speaking and comprehension of the spoken word and in reading and writing. Simple techniques for locating the language centers of the brain would be an advantage in developing education plans and teaching strategies for students with each of the possible hemispheric dominance patterns.